Conventionally, a so-called tandem type image forming apparatus in which photosensitive bodies corresponding to respective colors of yellow, magenta, cyan, and black are disposed in parallel in a horizontal direction has been known.
In this tandem type image forming apparatus, respective color toner images are formed on the corresponding photosensitive bodies generally at the same time, and are sequentially transferred from the photosensitive bodies to the sheet which sequentially passes each photosensitive body, so that the color image can be formed generally at the same speed as in the black and white image forming apparatus.
As such a tandem type image forming apparatus, for example, an image forming apparatus, in which four photosensitive bodies, a corona charger arranged around each of the photosensitive bodies, and a cleaning device are provided as a integral photosensitive cartridge, and it can be attached to and detached from the apparatus main body, and further, the developing unit which is an attachment to the photosensitive body is also detachably attachable to the photosensitive cartridge, has been proposed.
In this image forming apparatus, a black developing unit, and a cyan-magenta-yellow developing unit are detachably and replaceably attached to the photosensitive cartridge. When each developing unit is attached to the photosensitive cartridge, a fixed lever provided to the photosensitive cartridge pivots to fix the developing unit to the photosensitive cartridge.
Moreover, in an image forming apparatus, the developing unit described below has been also known. The developing unit is provided with a developing roller for feeding a toner to the photosensitive body. When the developing unit is attached to the photosensitive cartridge, an elastic member such as a spring provided to the photosensitive cartridge presses the developing unit, and in turn the developing roller is pressed to the photosensitive body with a predetermined pressing force.
However, the pressing force of the elastic member applied to the developing unit is gradually reduced by degradation of the elastic member with time. Accordingly, the pressing force of the developing roller to the photosensitive body is reduced, and then the toner supply from the developing roller to the photosensitive body is reduced, which may degrade the development quality.